<!-- 
[水平垂直居中的几种方式:]
    - 行内元素:
        text-align: center;
        line-height: height;
    - 块级元素:
        1. position: absolute; => top,bottom,left,right: 0; => margin: auto;
        2. flex布局
        3. position: absolute; => left:50%;top:50% => translate(-50%,-50%);
 -->